MULTIWAVELENGTH APPEARANCE OF VELA JR.: IS IT UP TO EXPECTATIONS? Anatoli F. Iyudin1, B. Aschenbach2, V. Burwitz2, K. Dennerl2, M. Freyberg2, F. Haberl2, and M. Filipovic3 1Skobeltsyn Institute of Nuclear Physics, Moscow State University, Vorob’evy Gory, 119992 Moscow, Russian Federation 2Max-Planck-Institut fur¨ Extraterrestrische Physik, Postfach 1312, D-85741 Garching, Germany 3CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW1710, Australia; University of Western Sydney, Locked Bag 1797, Penrith South, DC, NSW 1797, Australia ABSTRACT of the nearest supernova in recent history [3] with a best estimate for the distance of 200 pc and an age of 680 years. Vela Jr. is one of the youngest and likely nearest among the known galactic supernova remnants (SNRs). Discov- The detection of 44Ti in Cas A [17, 18] has been sup- ered in 1997 it has been studied since then at quite a few ported by the Beppo-SAX measurements of the ∼68 and wavelengths, that spread over almost 20 decades in en- ∼78 keV X-ray lines [50] which are produced in the first ergy. decay of 44Ti in the decay chain 44Ti!44Sc!44Ca, and recently by IBIS/ISGRI [38, 51]. The detection of the Here we present and discuss Vela Jr. properties re- 1.157 MeV γ-ray line from Cas A was the first discovery vealed by these multiwavelength observations, and con- of 44Ti in a young galactic SNR, and as such it provides front them with the SNR model expectations. an essential calibration of nucleosynthesis model calcu- lations. With the discovery of RX J0852.0-4622/GRO Questions that remained unanswered at the time of pub- J0852-4642 we may have a second example, which still lication of the paper of Iyudin et al. [25], e.g. what needs the confirmation by independent measurements. is the nature of the SNR’s proposed central compact SN1987A might be the third case of strong 44Ti line source CXOU J085201.4-461753, and why is the ISM emission, which can be probed in the future by more sen- absorption column density apparently associated with RX sitive γ-ray instruments. J0852.0-4622 much greater than the typical column the Vela SNR, can be addressed using the latest radio and Multiple observations of Vela Jr. in the radio X-ray observations of Vela Jr.. These results, as well as band confirmed the shell-like structure and supported other related questions we will be addressed in the fol- the identification of RX J0852.0-4622 as a SNR lowing. [8,11,12,13,14,16,46]. Furthermore a good correlation between X-ray and radio brightness was found (c.f. Fig. Key words: galactic supernova remnants; radio, optical, 1). and X-ray emission; nucleosynthesis; γ-ray lines. X-ray spectra of this SNR taken with ROSAT, ASCA, Chandra and XMM-Newton are well described by a 1. INTRODUCTION power law [3,4,22,33,44,48]. The best-fit with a single power law results in an interstellar absorption column density of 3.9×1021 cm−2 which is at least a factor of RX J0852.0-4622, referred to as six higher than the highest value of ∼6×1020 cm−2 ob- G266.2-1.2 in Green’s SNR catalogue served anywhere else in the Vela SNR [31] for the thermal (http://www.mrao.cam.ac.uk/surveys/snrs/), and oc- emission fit. casionally as Vela Jr., located at the south-eastern corner of the Vela SNR [2] was one of the discoveries of the Based on the ROSAT data Aschenbach [2] suggested the ROSAT all-sky survey. It is also the second brightest presence of a central point source, which later was con- excess in the ∼6 years all-sky map of COMPTEL in firmed by Chandra measurements [34]. The spectrum the 1.157 MeV line emission, called GRO J0852-4642, of the proposed compact central source (CCS) CXOU that coincides with RX J0852.0-4622 [19, 20]. Since J085201.4-461753 suggests a neutron star as the emit- the 1.157 MeV line of 44Ti is exclusively produced ter [23,5,6], which is supported by the absence of any in supernovae it is very likely that RX J0852.0-4622 optical counterpart brighter than R∼26 [30]. If this ob- and GRO J0852-4642 are the same object which was ject is the compact remnant of the supernova which cre- created in one supernova explosion. The combined ated RX J0852.0-4622 the supernova was of the core- analysis of the X-ray data and the γ-ray data led to the collapse type. Like for the north-western rim the absorb- suggestion that RX J0852.0-4622 could be the remnant ing column density of (3.450.15)×1021 cm−2 inferred wards the CCS of around ∼2.5×1021 cm−2 is expected which is not far from the columns measured for the CCS [6]. Concerning the age, there has been the suggestion, that the supernova which led to RX J0852.0-4622 is responsi- ble for a previously unidentified spike in nitrate concen- tration measured in an Antarctic ice core. The precipita- tion occurred around the year 1320. Other nitrate spikes could be associated with historical supernovae. One of the issues in this context is of course the detection of the 1.157 MeV line, because together with the 44Ti yield it dominates the estimate of the age. To get more insight into the questions of distance, age and progenitor of this SNR observations of the X-ray bright rims and of the central region were carried out by XMM- Newton [5,6] and by Chandra [4,23,33,34]. Figure 1. The Parkes image (colour) of RX J0852.0- 4622 overlaid with ROSAT PSPC contours (E>1.3 keV). The synthesized beam of the Parkes observations is 50 (lower left corner) with an r.m.s noise (1 σ) of 10 mJy. X-ray contours (white) are 0.8, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.3, 2.6 and 3 in units of 10−4 ROSAT PSPC counts s−1 arcmin−2. from the spectrum of CXOU J085201.4-461753 is signif- icantly higher by a factor of about six (Kargaltsev et al. 2002) than the column densities of ∼6×1020 cm−2 mea- sured for the Vela SNR by Lu & Aschenbach [29], which suggests that at least CXOU J085201.4-461753 and pos- sibly RX J0852.0-4622 are at a greater distance than the Vela SNR, which is located at a distance of 390100 pc, as was recomended by Cha & Sembach [7] based on ob- servations of 60 O stars. Pozzo et al. [36] concluded that the distance to RX J0852.0-4622 is ≈43060 pc, which is consistent with the distance to the Vela OB2 associa- tion that was derived to be 42030 pc by Woermann et al. [52], or with the distance to the Trumpler 10 (Tr 10) Figure 2. XMM-Newton MOS1/2 false color image of the OB association that was derived by de Zeeuw et al. [56]. inner 30 arcmin central region of RX J0852.0-4622 (red: 0.3-0.75 keV; green: 0.75-2 keV, and blue: 2-10 keV). The On the other hand, Redman et al. [37] suggested, as a central bright source is CXOU J085201.4-461753. Other possibility, that the optical nebula RCW 37 was gener- 18 point-like sources, marked by circles, are detected in ated by the blast wave of RX J0852.0-4622 impacting the the field of view. The angular bin size of the image is 4 shell of the Vela SNR, i.e. RX J0852.0-4622 is embed- arcsec [6]. ded in the Vela SNR. Moreover, the X-ray spectrum of the rather bright, nebular region C (Fig. 1), which is slightly eastward of CXOU J085201.4-461753 (Fig. 2), has an 2. RADIO OBSERVATIONS appearance typical of the Vela SNR shrapnel spectrum [25,26], but has an absorption column of ∼5.0×1021 cm−2 (Iyudin et al. 2006, in preparation), that is even Radio observations of RX J0852.0-4622 starting with higher than the column to the CCS. This fact, among oth- the publication of Dubner et al. [10] and on the Vela ers, suggests a high degree of clumpiness of the ISM to- and Gum Nebula regions by Yamaguchi et al. [55], wards Vela Jr. along different line of sights in this com- Moriguchi et al. [31] shed light on the SNR immediate plex region containing a number of molecular, HI and environment, demonstrating the presence of a multitude CS clouds [10,31,55,57]. We note, that this high absorp- of different clouds in the ISM surrounding the SNR. Es- tion column is derived for the line sight very close to the pecially noteworthy the paper by Dubner et al. [10] that line sight towards the CCS. If the column of ∼5.0×1021 shows HI clouds of an angular size comparable to that cm−2 corresponds to the thickness of two ejecta shells or of RX J0852.0-4622 whose boundary is likely to be in- two walls of the wind bubble, an absorption column to- teracting with the RX J0852.0-4622 blast wave (see Fig 3). The presence of many point-like X-ray sources in the vicinity of the forward shock can be used to demonstrate the increase of the absorbing column along the radii go- ing from the center of the SNR to the point source No.